Patent GB-680746-A (GB)
Title: Voice frequency signalling in telecommunication systems
IPC: H04Q1/45
Filed: 1950-10-27
Abstract: 680,746. Voice frequency signalling. STANDARD TELEPHONES & CABLES, Ltd. Oct. 27, 1950 [Nov. 5, 1949], No. 26220/50. Class 40 (iv). Interexchange signalling for the most part is carried out by the interruption of a normally transmitted frequency, different frequencies being used for the two directions, but release is signalled by the simultaneous transmission of both frequencies from one end. Setting up the connection. The circuit of the east terminals of the trunk is identical with that of the west terminal shown, with the exceptions that the frequencies of the oscillators O<SP>1</SP>, O<SP>2</SP> are interchanged as also are the band-pass frequencies of the filters BP<SP>1</SP>, BP<SP>2</SP>. In the idle condition the est terminal transmits frequency F<SP>1</SP> from oscillator O<SP>1</SP> via the filter BP<SP>1</SP> to line. At the east station the oscillation is passed by filter BP<SP>2</SP> to valve V<SP>1</SP> whose output is rectified to bias back valve V<SP>2</SP> to keep the impulsing relay AR unoperated. Similarly, frequency F<SP>2</SP> is transmitted from east to west. When the trunk is seized over the east selector SS<SP>1</SP>, PR operates to the loop over LA, LB to disconnect the oscillator O<SP>1</SP>, whereupon AR comes up in the east terminal. PR then repeats the next digit received over LA, LB as tone pulses to which AR (east) responds to set the selector SS<SP>2</SP> in known manner. A loop is then extended over DR to LA, LB, over which further digits are repeated by AR to complete the connection. Ringing tone &c. have frequencies outside the stop band (Fl to F<SP>2</SP> inclusive) of the filters BS through which they are reverted. When the called subscriber answers, current reversal over the loop LA, LB (east) operates the polarized relay DR which disconnects oscillator O<SP>1</SP> from the line, thereby causing AR (west) to operate and bring up HAR. The latter removes the band-stop filter BS from the speech path and also brings up VR which reverses the polarity fed to LA, LB as a supervisory signal. At the east terminal DR brings up HAR to remove filter BS and operate VR. In both terminals HAR connects the biasing circuit of V<SP>2</SP> to the tuned circuit L<SP>1</SP>C<SP>1</SP> (instead of R<SP>2</SP>) which is tuned to the difference frequency F<SP>1</SP>-F<SP>2</SP> and presents low impedance to frequencies F<SP>1</SP>, F2. Release. When the called subscriber (east) hangs up DR restores, releases HAR and brings up HBR. DR connects frequency F<SP>2</SP> to line and HBR connects up F<SP>1</SP> and these are intormodulated in the valve V<SP>1</SP> (west) to produce the difference frequency F<SP>1</SP>-F<SP>2</SP> which is tapped off the tuned circuit L<SP>1</SP>C<SP>1</SP> and rectified to back bias V<SP>2</SP>. Relay AR is thereby released followed by HAR. The latter disconnects filter BP1 from the input to the valve V<SP>1</SP> so that the valve receives F<SP>2</SP> only, but since HAR has reinserted R<SP>2</SP> into the output circuit of V<SP>1</SP>, valve V<SP>2</SP> remains back-biased to keep AR unoperated. HAR releases VR which reverses the polarity on LA, LB as a supervisory signal. When the calling subscriber hangs up PR releases. When VR (east) is released by HAR, HBR follows and AR is released by the transmission of F<SP>1</SP> from the west terminal when PR releases. Similarly, if the calling party hangs up first PR (west) operates HBR and releases HAR. Both F<SP>1</SP> and F<SP>2</SP> are then sent to the east terminal and release proceeds as before except that when VR (east) releases it disconnects DR from the loop LA, LB as a supervisory signal. Voice immunity. During the setting up of the connection no voice currents can pass the band-stop filters BS and so enter the signalling system via filters BP<SP>1</SP>, BP<SP>2</SP>. If during conversation a combination of F<SP>1</SP> and F<SP>2</SP> occurs AR at the near end is released followed by HAR which inserts the band stop filter BS to prevent onward transmission of these frequencies. Thus at neither end is VR de-energized sufficiently long for it to release. The difference between F<SP>1</SP> and F<SP>2</SP> is normally arranged to be outside the transmitted band.